st: tostring for large double type variables?

I'm appending files over multiple years where the variable type
changes over time. I have a variable that's a long string of numbers
(about 45 numbers) in some years, but which is stored as double in
other years. I'd like to be able to store the variable as either a
string or double in the appended file, but I can't figure out a way to
use the tostring command that doesn't truncate the double variable.
Is there an easy way to preserve the information and string the variable?
Thanks,
Paul
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